1. Field of the Invention
The present invention describes a method of applying microdrops of solder to hard to solder substrates, particularly aluminum and indium tin oxide.
2. Background of the Prior Art
Microelectronic semiconductor devices are useless without electrical contacts between electrical circuits and other devices. Electronic contacts and connections must be suitable and reliably made. In general, wafers containing multiple integrated circuits (IC's) or individual dies require a number of expensive, time consuming, environmentally unfriendly steps because the substrate will not directly accept solder. In general, the goal of the electronics industry is to start with an aluminum or aluminum alloy metallized silicon surface and arrive at individual solder "bumps" at particular locations in good mechanical, electrical and thermal contact with the aluminized surface of a wafer or die. Surprisingly, the invention permits solder bumps to be directly made in good electrical, thermal and mechanical contact at desired locations on aluminum or indium tin oxide (ITO) surfaces without the necessity of many of the intermediate steps normally required.
Most prior art processes first deposit a conductive layer over the whole surface of a wafer. The most common surface is usually aluminum. Further processing of wafers with individual IC's involves application of photoresist coatings with openings where individual interconnect pads are located. These pads are converted by plating the aluminum with sequential layers of other metals such as nickel, copper and finally solder. Unwanted plated areas are removed by etching. The remaining solder is supplied with flux and heated to reflow the solder into balls with the aid of surface tension to create "bumps" which serve to make electrical and mechanical connections to other parts. The resulting solder bumps must be inspected and tested because the existence of multiple process steps significantly increases the chances of failure. Even freshly deposited aluminum is hard to solder. Flux is used as a vehicle for dissolving the oxide and allowing solder to stick and bond.
Soldering directly onto aluminum without flux has been a goal in the electronics industry for decades and the authorities all recognize that there is no practical way to do it. People believe oxides on aluminum interfere with adhesion and wetting of solders. Use of flux to dissolve the oxide is undesirable in microelectronics fabrication because its presence after soldering is highly undesirable for a number of reasons, not the least of which is corrosion.
The other of the most frequently used hard to solder substances is indium tin oxide (ITO) which is used in displays. ITO is used in thin film format for the connectors of flat panel displays. It has sufficient conductivity and is transparent to visible light. Displays have glass sheets bonded together with the area between the sheets filled with liquid crystal material that make up the displays. ITO is a way to apply voltages to areas of the glass. There are a large number of electrical connections (lines) and each has to be connected to IC's distributed around the edges which drive it. Temperature limitations are of concern because of the materials used in the displays. Consequently, low melting In/Sn solders are employed. The usual method used to interconnect to ITO is to deposit metal pads onto the ITO and interconnect to the metal pads. Some methods require applying a solderable metallized coating onto the ITO in a vacuum where the metallic coating is evaporated or "sputtered" with masking to control deposition. A process that would allow for direct soldering to the ITO would be a major cost saving in the fabrication of flat panel displays or other optical devices that use ITO as the conductors.
A typical process for creating solder bumps has the following characteristic steps although there are a number of additional operating steps as well in getting from one main characteristic step to another. A process typically includes 1.) an aluminum or aluminum alloy metallized surface; 2.) sputtering, evaporation or plating of an adhesion/barrier metal such as Cr, TiW, etc.; 3.) deposition of a solderable surface by plating or evaporation of solderable metal, such as Cu, CuNi, Ni, etc.; 4.) deposition of a protective layer, such as Au, etc.; 5.) solder deposition such as by plating, printing, etc.; and 6.) reflow of the solder with the aid of flux to form bumps. In the invention described herein, characteristic steps 2, 3, 4 and 6 are eliminated.
Of the prior art processes, evaporation can provide bumps with the best uniformity and composition in volume production. The most commonly used evaporated bumps are those based on Pb/Sn (e.g., the IBM C4 process) deposited on a wafer through a molybdenum metal mask. The molybdenum metal masks are first aligned to the bond pads on the wafer and clamped. For interconnect systems such as C4, this metallization consists of chromium, copper, and gold. Following metallization the wafers and shadow mask assembly are transferred into a solder evaporation system. Here a known composition and volume of solder is evaporated onto the bond pads. The shadow mask is then removed. For several decades 100 micrometer diameter bumps on a 250 micrometer pitch have been demonstrated in manufacturing environments with this method. A limitation influencing the minimum size and pitch capability of evaporation solder bumps is the metal mask technology. This process can be characterized by the following steps: 1.) provide an aluminum alloy pad; 2.) deposit chromium; 3.) deposit copper; 4.) deposit is flash of gold; 5.) deposit a thick layer of solder; and 6.) reflow the solder to form bumps. There are also wet electroplating processes which have a similar number of characteristic steps and electroless processes which mimic the above. Knowledge of an improved solder that would wet glass, U.S. Pat. No. 5,120,498, has not resulted in a successful application to solder bumping despite the fact that the patent has been public since 1992.